Here's how a major new influenza pandemic might unfold. Let's imagine that a wild duck on a migratory path in China passes a strain of influenza to farm chickens via the fecal-to-oral route. The ducks may land in ponds or near chickens being raised for food, defecate, and release new strains of avian influenza. The new strain enters the local environment, where previously existing strains of influenza are still being exchanged among the dense population of chickens (thousands of birds can often be found in one large egg-production operation). On these same rural farms, the chickens may be in close proximity to pigs and transmit the virus—or almost as likely, multiple viruses—to the pigs.
The pigs in turn serve as a reservoir for the viruses—literally becoming a culture medium where the multiple strains of the virus can mix their RNA with one another. Within the tens or hundreds of thousands of recombinations that take place, it takes only one or two to emerge that appear to be infectious for humans. The strains may be similar to ones that emerged only a few years ago.
Why do epidemiologists single out China when other places around the world, such as India, also have extremely high population densities? The reason is that high population density in and of itself is not enough to generate or sustain the development of new strains of influenza. China is one place in the world where humans, birds, and pigs come into close contact with each other on a more or less continuous basis and where duck migratory “overflies” are routine and well established.
This phenomenon is less likely to occur in India, since pigs are not raised en masse because of the different climate zone and cultural and reli-